How do you create your own weather patterns for the area or are you just stuck with the original in game ones?

On the menubar, go to World>Regions... Choose new and make a name for the region. Then edit the weather chances. Note: They are based on a percent. If it says 10 in the box next to "Rainy", it means there will be a ten percent chance of rain in that region. This also means that all of them put together must add up to 100.